This invention relates to antennas and more particularly to a single multipurpose antenna structure.
Because of their higher speed, deeper submergence, and the increasing need for a more covert operation, modern submarines require communication facilities that are reliable, durable, and capable of operation with the lowest practical detectability. This is especially true for antennas.
One means towards achieving these needs is to decrease the number of required individual shipboard antennas by combining them into a single multipurpose antenna structure wherein the highest obtainable efficiencies are provided compatible with a practical size, form and ease of deployment.
A multipurpose antenna for submarine use embraces the transmission and reception of radio signals in various sectors of the frequency range from VLF (very low frequency) upwards through UHF (ultra high frequency). For practical reasons, the various antennas are stacked above one another within a hydrodynamically-shaped radome at levels dictated by the performance requirements of each individual antenna. Accordingly, the high-powered MF/HF (medium frequency/high frequency) transmitting antenna is relegated to the bottom-most level of the stack of antennas.
In order to serve the various antennas above the MF/HF antenna, their signal wires or cables must pass around or through the high-powered MF/HF antenna. When such conductors are passed around the MF/HF antenna, they must be enclosed in a shielding conduit and be as far removed from the MF/HF antenna as is physically practical because of the tendency for electrical coupling detrimental to MF/HF antenna performance. This displacement is severly limited in a submarine antenna.
To reduce the effects of electrical coupling of the service conductors which is detrimental to MF/HF antenna performance, a mode switch can be used to disconnect all of these conductors for optimum MF/HF performance when it is required. The mode switch has two states: multifunction, mean all functions available, and MF/HF only. The MF/HF antenna efficiency in the multifunction mode was 10 decibels less than desired, while the MF/HF only mode provided adequate efficiency. However, this is not completely desirable since all services above the MF/HF antenna would be rendered useless should the mode switch fail in an open position, and optimum MF/HF performance would not be had should the mode switch fail in the closed position.
Should a conduit of service conductors be passed directly through the MF/HF antenna rather than around it, it would tend to couple to the antenna to cause unacceptable performance or to absorb sufficient radiation so as to destroy the use of the conductors within it through overheating. To overcome this phenomenon, wherein a centrally-located conduit tends to couple into the radiated field of the antenna, and, in turn, itself tends to radiate because of mutually-induced current, the use of an isolator can minimize the conduit current. The isolator can be a tunable circuit in concert with the frequency range of the antenna and in series with the conduit to ground so as to effect a very large impedance to the flow of conduit current.
Because of mutual coupling a tunable MF/HF antenna of this type will demonstrate a very high voltage between parts of the radiating antenna and the conduit or, in reality, between the traveling short and its drive shaft since practical design readily permits a drive shaft to be concentric about the conduit. In high-power transmitting antennas, this voltage can be very high, for instance, in the order of 30,000 to 40,000 volts for a 2-KW (kilowatt) output, and of fairly high frequency, such as 2 to 30 megahertz. In submarine applications, practical considerations require the spacing between these elements to be quite limited.